A variety of physical and chemical deposition procedures have been used to prepare aluminum (Al) films. These methods are of interest, in part, because thin films of aluminum have many uses due to their high electrical conductivity, high reflectivity, mechanical strength, and their resistance to chemical attack. There is much current interest in generating thin films of aluminum using chemical vapor deposition (CVD), particularly resulting from applications in the microelectronics industry. In a typical CVD process, organoaluminum precursors are volatilized and then decomposed to yield aluminum, which is deposited as a film on the target substrate.
The generation of aluminum films using triisobutylaluminum (TIBA) has been reported. Typical deposition conditions generally require relatively high substrate temperatures, however. For example, the low pressure CVD of aluminum on silicon or other substrates using TIBA requires a substrate temperature of 260.degree. C., TIBA temperature of 45.degree. C., with argon as the carrier gas at pressures up to 1 torr. These conditions resulted in Al deposition rates of up to 0.15 .mu.m/min. See M. J. Cooke et al., Solid State Technol., 25, 62 (1982). The films produced by such methods have low resistivities (2.8 to 3.5 .mu..OMEGA.-cm) and other properties that are comparable to the properties of aluminum films prepared by evaporative techniques. See R. A. Levy et al., J. Electrochem. Soc., 131, 2175 (1984). However, a major disadvantage observed using TIBA is the rough morphology of the Al film, which leads to poor reflective properties.
Improved aluminum film uniformity, particularly on nonmetallic surfaces such as silicon, has been obtained by pretreating the substrate surface with TiCl.sub.4 prior to deposition of aluminum using TIBA as a precursor by chemical vapor deposition. See R. A. Levy et al., J. Electrochem. Soc., 134, 37C (1987); M. J. Cooke et al., vide supra; M. L. Green et al., Thin Solid Films, 114, 367 (1984); and R. A. Levy et al., vide supra. Although the aluminum films are more uniform, they may not be highly reflective. Recently, a method of preheating TIBA to a temperature of 230.degree. C. with the substrates at 400.degree. C. has produced aluminum films with reflectivities of 90% and epitaxial Al&lt;111&gt; growth on Si&lt;111&gt;. See T. Kobayashi et al., Jap. J. Appl. Phys., 27, L1775 (1988) and T. Kobayashi et al., Abstracts of Papers, Fall Meeting, Boston, MA; Materials Research Society: Pittsburgh, Pa., E9.47 (1988).
Other sources of aluminum have been used as precursors in aluminum plating processes. For example, D. L. Schmidt et al., U.S. Pat. No. 3,462,288 (1969), describe the use of aluminum hydrides such as AlH.sub.2 Cl, LiAlH.sub.4, and AlH.sub.2 (i-C.sub.4 H.sub.9) with a decomposition catalyst in an electroless solution plating process. In C. B. Roberts et al., U.S. Pat. No. 3,787,225 (1974), aluminum hydride-ether complexes were contacted with trimethylamine vapor in the presence of a decomposition catalyst.
A series of stable, volatile donor-acceptor complexes of alane (AlH.sub.3) have been known for many years. They can be generally represented by D.AlH.sub.3, and can be readily synthesized in one step from LiAlH.sub.4. These donor-acceptor complexes of alane are air sensitive, but they are not pyrophoric, as are the trialkylaluminums. Among the known donors (D) are Me.sub.3 N, Et.sub.3 N, Me.sub.3 P, Me.sub.2 S, and tetrahydrofuran (THF). See, for example, E. Wiberg et al., Hydrides of the Elements of Main Groups I-IV, Elsevier: Amsterdam, Ch. 5 (1971); and R. A. Kovar et al., Inorg. Synth., 17, 36 (1977). Trimethylamine is unique among these donors in its ability to form a bis complex with alane, i.e., (Me.sub.3 N).sub.2 AlH.sub.3.
The use of amine-alane complexes for the vapor phase deposition of aluminum have been described in T. P. Whaley et al., U.S. Pat. No. 3,206,326 (1965), and in D. R. Carley et al., U.S. Pat. No. 3,375,129 (1968). These methods, however, do not produce mirror-like coatings. Rather, less reflective "shiny" and "metallic" surfaces result. Laser-induced deposition of aluminum using amine-alane complexes has been disclosed in T. H. Baum et al., Abstracts of Papers, Fall Meeting, Boston, Mass.; Materials Research Society: Pittsburgh, Pa., B4.12 (1988).
Therefore, a need exists for a process to deposit a mirror-like coating of aluminum having low resistivity using precursors that will permit the use of substantially lower deposition temperatures in the CVD process.